a) Field of the invention
The present invention relates to a process of treating at a high temperature, a dross containing a metal in order to recover the metal.
More particularly, the invention relates to a process for treating a dross containing a metal, in particular aluminum, in a rotary or oscillating furnace, in order to recover the metal from the dross.
In the following description, only aluminum has been specifically mentioned as metal to be recovered. If the recovery of this particular metal from the dross floating on the top of an aluminum bath is actually one of the most important applications of the process according to the invention, it is worth mentioning that the invention as described and claimed hereinafter is not exclusively restricted to the recovery of aluminum. As a matter of fact, the process according to the invention is also useful for the recovery of other metals, in particular nonferrous metals, from drosses containing them.
b) Brief description of the prior art
It is a conventional practice to transfer the aluminum produced by electrolytic reduction of alumina in a furnace (usually called "holding furnace"), where the metal is eventually remelted and held in melted form before being transferred to another location where it is laminated and/or otherwise treated. In the holding furnace, a foam-like mixture of aluminum and alum called "dross", forms on the surface of the bath. The dross that may represent up to 5% of the melted metal bath, contains a substantial amount of aluminum, in the order of 50%. Of course the recovery of the aluminum in the dross is very important recovery can be carried out by treating the dross in a furnace at a high temperature. For this purpose, several furnaces nave been devised and are presently being used in aluminum plants. To the Applicant's knowledge, the existing furnaces are exclusively heated with fuel- or gas-operated burners, or with plasma torches.
The use of fuel- or gas-operated burners for heating the dross in a furnace in order to recover the aluminum contained therein, has the major drawback of requiring the addition of salt fluxes such as NaCl or KCl, in order to increase the percentage of aluminum recovery.
The use of a plasma torch as suggested in Canadian patent No. 1,255,914 issued on Apr. 20, 1990 to ALCAN INTERNATIONAL LTD., permits the above mentioned drawback to be overcome. Indeed, the use of the plasma torch creates higher temperatures in a furnace and thus avoids the necessity of adding salt fluxes. However, the use of the plasma torch has a major drawback, due to the way the existing torches are cooled with water. Indeed, in case of a water leak, heavy explosions may occur and result in serious accidents.
In the above mentioned Canadian patent No. 1,255,914, it is essentially suggested to operate the plasma torch in the non-transferred arc mode. It is however mentioned in page 6 of the specification, that under certain circumstances, it may be possible to operate the plasma torch in the arc transferred mode. This patent does not explain however how such an arc transfer can be reduced to practice, nor the advantages that may derive therefrom.
In both cases anyway, use is made of a plasma torch that must necessarily be cooled thereby water, with creating the drawback discussed above.